The use of automatic protection switching in the field of telecommunication transmission is employed as a means of reducing the out of service time where signals must be transmitted over span lines, which lines include repeater amplifiers. In telephone systems, communication occurs over pulse code modulated (PCM) transmission lines which may multiplex a number of terminals onto a common PCM channel. Accordingly, in such systems, communications between central offices occur over span lines which essentially consist of multiwire cables containing repeating amplifiers at intervals. The main function of such span lines is to maintain the data within acceptable amplitudes during transmission between offices.
It is, of course, possible that a span line or a repeater contained in such a line exhibits a failure and hence, the line becomes inoperative. Therefore, the prior art has determined that it is desirable to switch a defective span line to an operative span line in order to maintain communications. As indicated, such an interruption in transmission line due to outside equipment failure or cable breakage is thus avoided by automatic transfer of the signal to a working spare line.
The prior art has been cognizant of such problems and has employed systems which operate to monitor the quality of the signal and to provide automatic transfer to a spare line upon detection of signal degradation. Upon removal of a fault condition, the data would then be transferred back to the original line.
For an example of such prior art techniques, reference is made to U.S. Pat. No. 3,715,503 entitled AUTOMATIC TRANSFER ARRANGEMENT FOR A TELEPHONE SYSTEM issued in February, 1973 to Jungbluth et al.
In any event, with the increasing demand for high speed operation which is attendant with high frequency of transmission, the information capacity is much greater and hence increased reliability is necessary in order to operate such systems in a reliable and efficient manner. Therefore, the need for protective switching in regard to span lines becomes even more important. However, if one employs the lower frequency schemes of the prior art, the protection apparatus becomes extremely expensive while consuming a great deal of power. Accordingly, it is necessary to provide an alternative approach for high speed operation.
In particular, a modern day system may employ, as a transmission medium, optical fibers coupled to and which operate with suitable optical transmitters and receivers and which function as span lines. As a transmission medium, optical fibers have a number of unique advantages when compared with the conventional conductors. In regard to such advantages, is the ability to provide a system with a low total cost. In implementing such systems, only one fiber is required per direction of transmission. Hence, this feature makes optical fibers attractive for use in crowded ducts. Furthermore, terminals and/or repeaters can be spaced at greater intervals because of the reduced signal attenuation/degradation in optical fibers. In this manner, the attenuation per unit length of optical fibers allows a longer repeater spacing than that employed in coaxial cables. In present day systems, such repeaters can be spaced between four to five miles or six to eight kilometers apart. In view of progress being made in present technology, the performance of such optical fibers will probably improve in the future and allow even greater spacing.
The optical fiber further permits reduced system cost based on its wide band width, low attenuation and the attendant reduction of cable size. Presently graded index fibers as manufactured by the Assignee herein can operate at bit rates varying from 1.544 Mb/s to 44.736 Mb/s. In fiber communications, there is also a virtual absence of crosstalk and hence, systems of various bit rates may be mixed in the same optical fiber cable. The fibers are completely immune to all forms of electrostatic or electromagnetic interference and are virtually unaffected by moisture, while providing a great deal of system security.
In any event, in employing such fibers in an optimum way, one must face various problems which are associated with fiber use. At the high frequencies of transmission, fiber optical systems typically employ single fibers as the transmission medium and a laser diode as the light source. Since it is difficult to transmit multilevel signs using laser sources, a binary signal is normally sent. Unless there are error check bits built into the frame format of the signal, there is no facility for in-traffic line error monitoring and therefore the line signal frequency must be increased to include the extra bits for error detection.
It is therefore an object of the present invention to provide an automatic protection switch for a span line employed in a high speed digital system, which system may include optical fiber span lines. It is a further object of this invention to modify the parity bit content in such a system to obtain in-traffic bit error rate monitoring and to further use those bits for bit error purposes to enable control of a protection switch for a span line.